Traci realizes that Dov couldn't see anything behind the counter from where he was standing. That's why he would have no idea if there was more than one person back there. The theory is that a second person was involved in the robbery. This additional suspect could have snuck out the back door with the gun. Andy and Sam track down the dead kid's friend. He's carrying a gun that matches with the slug found at the store. Jerry lets Dov know that he's off the hook. This was a clean shoot.
In random news, Traci learns that she got the detective rotation gig. Gail lures Nick into her car to lay down ground rules for their relationship. Sam lets Andy know that without her faith in a friend, Dov may have not been cleared. Noelle lets Frank know that she was scared for the baby during the ruckus on the street. That's why she'll do desk duty from here on out. As for Dov, he stops at the vigil taking place for the kid he shot. Then heads home after a long day on the job he loves.